The study utilizes two different stable isotopes of calcium to determine calcium kinetics. The fact that the isotopes are non-radioactive, they can be used in children and pregnant women. One isotope is given intravenously, the other by mouth. Since the isotopes have different atomic weights, the two calciums can be determined in the mass spectrometer simultaneously. Blood sampling is required for 12 hours only. Urine collections are made continuously every 8 hours for 2 weeks. Using the modeling program designed by Dr. Mones Berman, one can determine the size of the several calcium pools. The study had been amended to include the study of pregnant and lactating women. Our subject has been studied in the third trimester, during lactation and the study will be repeated 6 months after lactation has concluded. The Prader-Willi Syndrome continues in relation to the genetic components, evidense for altered metabolism and in the future calcium balance studies.